Hello...I have a JCM 800 2203 that I purchased back in 1981 and I wanted to replace all the tubes in the head after all these years. My amp has many hundreds of hours on it between practice and hundreds of gigs back all through the 80's. I assumed that I had the standard compliment of Four EL84's in the output stage of the head...Everything I've read stated as such, and all the You Tube Bias Procedures led me down the same path....SO I ordered a matched set of TAD 12AX7's (3), and four EL34's. Well, much to my suprise, when I took the back metel cover off the head, I did not find four EL34's...What was installed for power tubes back in 1981 was 2- 6550's, and 2- 6L6's. Both larger than the EL84's. Can anyone tell me why they (Marshall corp.) would do this...is there any special reason? I DO NOT want to change ANYTHING about the tonal characteristics of this amp. I would sure like a little enlightenment on the what's and the why's. I am truly facinated at discovering this..Thaks to all...Randy
Regarding your question we wouldn`t fit two 6550 and two 6l6 in the same amplifier as there bias requirements are different. You will need to have the amplifier checked by an engineer to see if the amplfiier has been altered to acommodate these valves, fitting EL34 straight into the amplifier can cause damage and will sound tonally different. Your amplifier may have been orginally set up to take 6550 power valves this will need to be checked by the engineer.